It has been known that the temperature characteristics and electric characteristics of an acoustic wave device are improved by providing a predetermined shaped protective film covering a comb-shaped electrode located on a piezoelectric substrate as disclosed in, for example, Japanese Patent Application Publication No. 2009-201168. There has been also known an acoustic wave device in which a frequency adjusting film is located on a protective film covering a comb-shaped electrode as disclosed in, for example, Japanese Patent Application Publication No. 2013-145930.
When a dielectric film covering a comb-shaped electrode and made of silicon oxide is provided to improve the temperature characteristics, a part of the dielectric film is removed to secure the electrical connection between the comb-shaped electrode and an external device. When the dielectric film is removed by dry etching, the manufacturing cost increases because the etching time of the dielectric film is long due to its relatively thick film thickness and it is difficult to process a plurality of wafers at the same time by dry etching. Thus, it is desirable to remove the dielectric film by wet etching. In this case, however, a wiring line to be connected to the comb-shaped electrode is damaged by being exposed to an etching liquid, resulting in decrease in yield ratio.